Lamp support arm



FIG. 1 is a perspective view from above of a lamp support arm in accordance with my design;

FIG. 2 is a side elevation view of the lamp support arm in accordance with my design;

FIG. 3 is a top plan view of the lamp support arm in accordance with my design; and,

FIG. 4 is a bottom plan view of the lamp support arm in accordance with my design.

The broken lines showing environmental structure are for illustrative purposes and form no part of the claimed design. 

The ornamental design for a lamp support arm, as shown and described. 